Development of progression of left ventricular hypertrophy has recently been described in children with hypertrophic cardiomyopathy. To determine whether similar changes in magnitude and distribution of left ventricular hypertrophy may also occur in adult patients with this disease, serial assessment of left ventricular wall thickness was obtained with M-mode and two-dimensional echocardiography in 65 patients with hypertrophic cardiomyopathy who were 23 to 50 years of age. Follow-up period was 3 to 6 years (mean 4). None of the study patients showed a substantial increase (more/less than 5mm) in left ventricular wall thickness. However, nine (14%) of the study patients demonstrated a substantial decrease of 5-9 mm in left ventricular wall thickness; wall thinning most commonly involved the anterior ventricular septum (7 patients), but was also identified in the posterior septum (6 patients), the lateral free wall (2 patients), and the posterior free wall (1 patient). In the nine patients who showed wall thinning, left ventricular end-diastolic diameter increased significantly (from 44 plus/minus 6 mm to 51 plus/minus 6 mm; p less than 0.001); however, in seven of the nine patients, absolute cavity size remained within normal limits (more/less than 52 mm) at the most recent evaluation. Eight of the nine patients with left ventricular wall thinning and relative cavity enlargement were severely symptomatic and one was mildly symptomatic. In conclusion, substantial progression of left ventricular hypertrophy was not identified in any of our adult patients with hypertrophic cardiomyopathy. Hence, if this phenomenon occurs in adults with hypertrophic cardiomyopathy, it is probably rare. Conversely, an important minority of adult patients with hypertrophic cardiomyopathy may show progressive left ventricular wall thinning and relative cavity enlargement which are usually associated with poor clinical prognosis.